In Frequency Division Duplex (FDD), a user needs to transmit channel state information to the base station side in the form of CDI (Channel Direction Information) and CQI (Channel Quality Information) as feedback, and the base station side reconstructs the channel using the CDI and CQI transmitted as feedback, and based on the reconstructed channel, performs precoding and the like to perform downlink transmission.
In a multi-antenna MIMO system, a user needs to transmit the channel state information to a single base station as feedback, and each state information includes large-scale channel state information and small-scale channel state information. The large-scale channel state information is mainly determined by a relative position between the user and the base station. Meanwhile, the small-scale channel state information indicates attenuation status between antennas of the base station and the user, and each element meets complex Gaussian probability variable with the average of “0” and variance of “1”.
Meanwhile, in a Coordinated Multi-Point (COMP) system, a user needs to transmit a plurality of channel state information of a plurality of base stations as feedback. Assuming that K coordinated points perform coordinated multi-point transmission/reception, the user needs to transmit the following channel state information of a plurality of base stations as feedback.H=[l1H′1,l2H′2, . . . , lKH′K]In addition, li, i=1, . . . , K represents large-scale channel state information of an ith base station, and H, i=1, . . . , K represents small-scale channel state information of the ith base station.
As an example in conventional techniques, in a channel state information feedback method of the Coordinated Multi-Point (COMP) system, assuming that K base stations join the coordination, a codebook is expressed by the following equation.C={Ci}, i=1, . . . ,2N 
In addition, Ci represents an ith codeword in the codebook, the codeword Ci includes quantization information respectively associated with K coordination joining base stations, and Ci can be expressed by Ci=[Ci,1, . . . , Ci,k]. N is the number of bits transmitted as feedback.
In the method, joint quantization is performed on the channel state information of multi-cell using a codebook C of a relatively large order, and the processing on the client side includes the following first to fifth steps.
In the 1st step, extracted is H′=[H′1, H′2, . . . , H′K] that is small-scale channel state information of multi-point.
In the 2nd step, extracted is li, i=1, . . . , K that is large-scale channel state information of multi-point.
In the 3rd step, acquired is CQI information (CQIi=li·|H′|) of multi-point, in which ·represents a dot product.
In the 4th step, the small-scale channel state information of multi-point is quantized using the large codebook to acquire CDI information. The rule of quantization is expressed by the following equation.
  CDI  =                    arg        ⁢                                                  ⁢                                                ⁢        max                    1        ≤        j        ≤                  2          N                      ⁢                                                C            j                    ·                      H                          ′              ⁢              H                                                                                          C            j                                    ·                                        H                          ′              ⁢                                                          ⁢              H                                                    
In the 5th step, a plurality of CQIs and a single piece of CDI information is transmitted as feedback, and a channel to reconstruct in the base station is expressed by the following equation.Ĥ=[CQI1·CCDI,1, . . . , CQIK·CCDI,K]
As another example, the channel state information feedback method of the Coordinated Multi-Point (COMP) system is actualized by joint quantization of multi-cell based on the large codebook, and the processing flow on the user equipment side includes the following first to fifth steps.
In the 1st step, extracted is H′=[H′1, H′2, . . . , H′K] that is small-scale channel state information of multi-point.
In the 2nd step, extracted is li, i=1, . . . , K that is large-scale channel state information of multi-point.
In the 3rd step, acquired is CQI information (CQIi=li·|H′|) of multi-point.
In the 4th step, complete channel state information of multi-point is quantized using the large codebook to acquire CDI information. The rule of quantization is expressed by the following equation.
  CDI  =                    arg        ⁢                                                  ⁢                                                ⁢        max                    1        ≤        j        ≤                  2          N                      ⁢                                                C            j                    ·                      H            H                                                                              C            j                                    ·                                        H            H                                        
In the 5th step, a plurality of CQIs and a single piece of CDI information is transmitted as feedback, and a channel to reconstruct in the base station is expressed by the following equation.Ĥ=[CQI1·CCDI,1, . . . ,CQIK·CCDI,K]
Further, as another example, in the channel state information feedback method of the Coordinated Multi-Point (COMP) system, quantization is performed independently for each cell based on a small codebook. It is assumed that the codebook is expressed by the following equation.C′={C′i}, i=1, . . . ,2N 
In addition, C′i represents an ith codeword in the codebook, N is the number of bits transmitted as feedback, and the selected C′i is associated with quantization information of any one of base stations. In the method, the channel state information of each cell is quantized independently using a codebook C′ of a small order, phase information of a plurality of cells is then quantized, and by combining, the channel state information of a plurality of cells is obtained. It is assumed that the codebook to quantize the phase information is expressed by the following equation.D={Dj}, j=1, . . . , 2M 
In addition, Dj is complex expression of a phase angle to quantize, and is expressed as follows:
            D      j        =          exp      ⁡              (                                                            sqrt                ⁡                                  (                                      -                    1                                    )                                            *                        ⁢            j            ⁢                                                  ⁢            2            ⁢                                                  ⁢            π                                2            M                          )              ,      j    =    1    ,  …  ⁢          ,      2    M      M is the number of bits required to quantize the phase angle.
In the channel state information feedback method for performing quantization independently for each cell based on the small codebook, the processing flow on the user equipment side includes the first to eighth steps as described below.
In the 1st step, extracted is H′=[H′1, H′2, . . . H′K] that is small-scale channel state information of multi-point.
In the 2nd step, extracted is li, i=1, . . . , K that is large-scale channel state information of multi-point.
In the 3rd step, acquired is CQI information (CQIi=li·|H′i|) of multi-point.
In the 4th step, the small-scale channel state information of each cell is quantized using the small codebook to acquire CDI information. The rule of quantization is expressed by the following equation.
            CDI      i        =                            arg          ⁢                                          ⁢          max                          1          ≤          j          ≤                      2            N                              ⁢                                                            C              j                        ·                          H              i                              ′                ⁢                                                                  ⁢                H                                                                                                              C              j                                            ·                                                H              i                              ′                ⁢                                                                  ⁢                H                                                                      ,      i    =    1    ,  …  ⁢          ,  K
In the 5th step, the multi-base station phase information is defined as G=[g1, . . . , gK], the codebook is quantized based on the phase, and the following quantization phase set candidates are combined.G′=[D1,j1, . . . ,DK,jK,],jk=1, . . . ,2M, k=1, . . . , K 
It is understood that the aforementioned quantization phase set G′ has 2MK kinds of combinations.
In the 6th step, a channel is reconstructed using the CQI, CDI and quantization phase set candidates of multi-cell, and the reconstructed channel is expressed by the following equation.Ĥ=[D1,j1·CQI1·CCDI1, . . . , DK,jK·CQIK·CCDIK]
In the 7th step, the reconstructed channel is compared with the complete channel state information of multi-cell, and the quantization information IPI (Inter-cell Phase Index) of multi-cell phase is acquired by an inclusive search. The rule of the search is expressed by the following equation.
  IPI  =                    arg        ⁢                                                  ⁢                                                ⁢        max                    1        ≤        j        ≤                  2          MK                      ⁢                                                              H              ^                        j                    ·                      H            H                                                                                            H              ^                        j                                    ·                                        H            H                                        
The phase index associated with each cell is expressed by the following equation.IPIi, i=1, . . . ,K 
In the 8th step, the CQI, CDI and multi-base station phase information is transmitted as feedback, and using the CQI, CDI and multi-base station phase information transmitted as feedback, the base station side reconstructs the following multi-point channel.Ĥ=[DIPI1·CQI1·C′CDI1, . . . ,DIPIK·CQIK·C′CDIK]
In each of above-mentioned methods, there is a problem that system throughput performance gain by a unit feedback amount is not high in any of the methods.